Rectify Boottime property

__mount gets called multiple times. Set the read-only property only upon
success since it can't be set multiple times. Selinux disallows modifying
read-only property and therefore the property holds a wrong value, the one
when __mount was called first even in case the call failed.

Bug: 224824099
Signed-off-by: Padmanabha V <padmanabha.v@sasken.com>
Change-Id: If2900fcf988b6225bb0f96586b00b97386ca8a81